Serine, catabolism dehydratase

Serine - Serine has many important biological roles, including the biosynthesis of phosphopholipids and cysteine. Serine also contributes activated one-carbon units to the pool of tetrahydrofolate coenzymes. Serine can be made in a variety of ways, including the way shown here and Figure 21.24. Serine is catabolized by conversion to glycine or by action of serine-threonine dehydratase (Figure 21.25). 

L-Serine and L-threonine dehydratases dehydrate and subsequently deaminate the amino add to the corresponding a-keto add. These enzymes are known to require pjn-idoxal-S -phosphate as a coenzyme. They can function in a biosynthetic or catabolic marmer (99). Both enzymes can cause problems for the whole-cell-based production of L-serine (100). 

Threonine can be broken down by tw o separate pathways. Serine dehydratase catalyzes the conv ersion of threonine to 2-ketobutyrate plus an ammonium ion 2-ketobutyrate is then converted by branched-chaln keto acid (BCKA) dehydrogenase to propionyl-CoA plus carbon dioxide. Propionyl-CoA catabolism is described later in this chapter. Threonine can also be broken down by a complex that has been suggested to be composed of threonine dehydrogerraseand acetoacetone synthase (Tressel ef al., 1986). Here, threonine catabolism results in the production of acetyl CoA plus glycure. 

Serine is one of the two hydroxyamino acids, the other being threonine. Serine has two major pathways of catabolism. The first, and apparently predominant, direction in many mammals is catalyzed by serine dehydratase, where water is removed between the alpha and beta carbons of serine. A rearrangement of the double bond forms an amino acid with spontaneous hydrolysis to form pyruvate and ammonia. Pyruvate then can be metabolized as discussed in previous chapters. This enzyme is primarily active in the liver, where the ammo-... 

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